Abstract

Abstract

Sugarbeet (Beta vulgaris L.) crops usually receive N fertilizer and sugarbeet tops are often returned to the soil following the harvest of storage roots. Residual sugarbeet fertilizer N and N in tops may reduce the amount of fertilizer N required for a subsequent crop. Field experiments at Davis, CA in 1980 and 1981 were designed to assess the contribution of these factors to a following wheat (Triticum aestivum L.) crop and to trace the partitioning of fertilizer N applied to sugarbeet, N in sugarbeet tops, and fertilizer N applied to the following wheat. The site was a Reiff loam, a deep alluvial coarse loamy soil with medium-textured substratum (Typic xerothents, Entisols, mixed, nonacid thermic). Sugarbeet, with and without 157 kg fertilizer N ha−1 and each with and without beet tops returned to the soil, were followed by wheat receiving four rates of fertilizer N (0, 62, 124, and 186 kg ha−1). The fertilizer used for both crops was stable, isotopically labeled 15N-depleted ammonium sulfate. Nitrogen from sugarbeet tops was traced in the wheat crop by returning sugarbeet tops containing labeled N to plots where sugarbeet had been fertilized with nonlabeled N and tops and storage roots removed. Residual sugarbeet fertilizer N was traced by additional plots of sugarbeet fertilized with more strongly labeled 15N-enriched ammonium sulfate. Of the 157 kg N ha−1 applied to the sugarbeet crop, 43% was taken up by tops and storage roots, 45% remained in the soil, primarily as organic N, leaving 12% not accounted for. Of the 45% (71 kg N ha−1) remaining in the soil only about 10% was taken up by the wheat crop, 66% remained in the soil, mostly as organic N, and 24% was not accounted for. When beet tops were returned to the soil, wheat straw and grain yield were maximid for this site with about 62 kg fertilizer N ha−1, but when tops were not returned, 124 kg fertilizer N ha−1 were required. From these applications, fertilizer N recovery by wheat was 82 and 61%, respectively. Sugarbeet tops contained, on average, 119 kg N ha−1 of which 27% was taken up by the wheat crop, 39% remained in the soil, mostly as organic N, leaving 34% not accounted for. Sugarbeet tops supplied the equivalent of 15 kg fertilizer N Mg−1 dry tops.

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